One of the mysteries in the study of hearing is how we are able to localize sounds accurately in reverberant environments in which the majority of sound energy is from different directions than the true source of sound. The reflected sound is not ignored by the auditory system;it colors the sound image and adds to the loudness and spatial width of the perceived sound. Nevertheless, as a result of the well-known precedence effect, localization is seriously affected only in the most extreme reverberant environments. The goal of our work is to achieve a better understanding of the precedence effect, the processes through which it operates, and its importance in understanding speech in noise. The first of three specific aims will consider how, and under what circumstances, the first wave of sound that strikes the ears controls the perceived direction of the auditory image, even if the remainder of the sound would lead to an ambiguous or different localization. The second aim is to characterize the fusion of sources and reflections into a single image that is localized near the original source of sound. These studies will investigate the hypothesis that fusion is enhanced as the listener constructs an internal model the acoustic spatial environment. The third aim is to determine how the precedence effect assists listeners in understanding speech in common situations in which multiple conversations occur simultaneously. .The studies will investigate the role of sound localization in understanding speech when the speech signal is degraded in ways relevant to listening with hearing loss, hearing aids and cochlear implants. In this way, the proposed research will help inform decisions that will improve speech recognition by hearing-impaired individuals wearing prostheses. Specifically, this research will determine which cues are critical to preserve and deliver for sound localization when designing or prescribing prosthetic devices.